Not sure why this was downvoted. These are excellent questions.
1- What I don't understand is if this is all decentralized who remove
from the DHT peer that no longer host a file content?
I think that DHT entries are regularly re-broadcast. So if a peer goes away, its DHT entries will no longer be broadcast and the network will forget about the data it provides unless some other node has it.
2- What prevent someone from storing large amount of data for free
inside the DHT?
Unless you re-publish or somebody else is interested in the data, it will vanish. The amount of data that you can store directly in a DHT entry is limited. So you can make other nodes store some of your data by putting data directly into DHT entries, but the effort outweighs the benefits.
3- What prevent someone from disrupting the network by adding large
number of invalid peer for a popular file.
I think there are some mechanisms envisioned in IPFS to protect the DHT against attacks. However, I don't think the current implementation is all that sophisticated. I don't think that current IPFS would deal well with a large scale distributed DDOS attack.
4- What prevent a bad actor from joining the DHT ring and not
following the routing protocol thus preventing discovery message from
reaching correct nodes.
I think a single node would be insufficient to do much damage, because a node will ask multiple peers. You would have to have multiple nodes to do significant damage.
But IPFS as it is now would not survive a sophisticated attack by state actors.
